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t ~l ~1 ~ t l r t , i ( ( r , \ ( ( \ ( \ \ I ( ( .. ; \ i , physical Feasibility Most structures, whether residential, commercial or industrial, are not designed to withstand hydrostatic pressure on the exterior walls, The principal reason more structures do not collapse during flooding is that water enters the structure equalizing inside and outside pressures. If the objective is to prevent water from entering a structure it is imperative that the structure be analyzed to insure that it can, withstand the pressures anticipated. Therefore when discussing physical feasibility the principal considerations are that 1), the exterior walls are impermeable or can be made so, 2) all openings below the design flood level can be closed, and 3) the structure can withstand the anticipated hydrostatic pressures including buoyancy. Watertightness - Structures with exterior walls constructed of brick, brick veneer, concrete and cement block are relatively impermeable and can be made more so by sealing exterior surfaces. Similarly basement walls are usually of concrete or cement block and basement floors of concrete and therefore relatively impermeable. Structures of these types of materials are particularly suited to keeping out water and the only adjustments necessary is to minimize seepage through walls and floors with sealants and temporarily or permanently closing doorways and windows. Structures with sidings such as wood, aluminum, sheet metal, or masonite on either a wood or steel frame are generally permeable and difficult to keep water out. Similarly structures on raised foundations with wood flooring are much more permeable than concrete slab-on-grade. Even for structures constructed of relatively impermeable materials the condition of the structure, and the number, location and size of openings influence the feasibility of providing closures. Masonry structures with extensive cracking or deteriorated materials may be little better than structures of permeable materials. Likewise, masonry structures with large and/or numerous openings lack the advantages associated with a structure with fewer openings. The most favorable condition for sealing and closing is a structure constructed of relatively impermeable materials, in good condition, and with few openings. Structural Adequacy - Assuming a structure can be made to exclude water, the next consideration is for its structural adequacy. When water is prevented from entering a structure the walls become subject to lateral hydrostatic forces which may cause failure by bending or shear, and the floors to uplift forces which may cause buckling or flotation. In addition, dynamic forces may be present if the flow velocity is great. Most structures are not designed to carry these forces and consequently are in danger of collapse or floating if the flood water rises too high. It is particularly difficult to analyze the capability of existing structures to resist these forces because of the general lack of knowledge about workmanship and materials used during construction and about the present condition of these materials. Analyses can be made where assumptions are made regarding the design and materials used, but they are only valid where the conditions assumed do exist. As part of this investigation structural analyses were made on the ability of walls and floors of various materials to withstand hydrostatic lateral and uplift pressures (6). Current construction practices for residential structures, new materials, and good workmanship were assumed. Generally these analyses confirmed analyses made by others (1, 2, 3) and are summarized below. 14